A metal laser-sintering technology has been known as a means for manufacturing a three-dimensional shaped object wherein the shaped object is obtained by repeating steps of (1) irradiating a powder layer of a metal powder with a light beam (e.g. directional energy beam such as laser beam) to form a sintered layer and (2) providing another powder layer on the sintered layer thus formed and irradiating it with the light beam to form another sintered layer. This technology makes it possible to manufacture the three-dimensional shaped object with a complicated profile in a short period of time. Particularly when a sufficient melting of the metal powder occurs by the irradiation of the light beam with high energy density, a sintered density of almost 100% can be achieved after the solidification of the melted metal powder. The resulting object with such high density can be subsequently subject to a finish machining treatment for smoothing a surface thereof. The shaped object thus obtained can be used as a metal mold for plastic molding.
In this regard, however, the metal powder used as a raw material for such metal laser-sintering is required to have different characteristics from those of another kind of powder-sintering process in which a powder compacting followed by a sintering of the compacted powder is performed.
For example, it is required for the metal powder to have a particle diameter smaller than the thickness of the powder layer to be irradiated with the light beam. Such smaller particle diameter provides a higher packing density of the powder as well as an improved absorption efficiency of the light beam upon producing the shaped object. This will lead to a higher sintered density and a smaller surface-roughness of the shaped object. On the other hand, when the particle diameter is too small, the metal powder tends to form the aggregated particles so that a packing density of the powder becomes lower, thus making it impossible to uniformly form a thin metal layer thereof.
In order to increase the strength of the shaped object, it is required that a contact area is large and a bonding strength is high between a newly formed sintered layer and a preceding and solidified sintered layer lying thereunder. In this case, even between the newly formed sintered layer and an adjacent solidified sintered layer, there is required a large contact area and a high bonding strength.
Furthermore, it is required that a top surface of the newly formed sintered layer does not have a significant bulge. The bulge with more than the thickness of the powder layer can interfere with the spread of the subsequent powder layer, making it impossible to form such subsequent powder layer.
Upon irradiating the metal powder with the light beam, the metal powder is allowed to melt partially or wholly. The melted metal powder is then solidified by a subsequent rapid cooling thereof. This results in a formation of a sintered material. When the melted metal powder has a high wettability, the contact area between the melted metal powder and the adjacent solidified sintered layer becomes larger, in which case a higher fluidity of the melted metal powder provide a less bulge. Therefore, it is desired that the metal powder, when melted, has not only a high fluidity but also a high wettability.
There is a possibility that the three-dimensional shaped object produced by the metal laser-sintering process has the residual metal powder adherent to the surface thereof, resulting in a rough surface of the object. Thus, in a case where the three-dimensional shaped object is used as a metal mold for plastic injection molding in which a high accuracy is required, such residual metal powder must be removed by carrying out a finish machining with a machining tool or the like. When the metal laser-sintering process is carried out by using a metal powder containing an iron-based powder with high hardness, the edge of the machining tool may wear out due to the hardness of the iron-based powder during the machining operation. Particularly when a narrow groove of the object is machined, it is required to use a machining tool having smaller diameter that is more prone to wear out and may undergo a chipping or breakage. Therefore, it is desired to use a metal powder which makes for a better machinability during the finish-machining operation and the like.
The three-dimensional shaped object thus obtained must have no significant crack on the outer surface thereof. In particular in a case where the three-dimensional shaped object is used as a metal mold for injection molding, it is desired that there is no micro crack in the inner structure of the object, considering that a flowing fluid is passed therethrough as a coolant.
In the light of the above, the applicant of the present invention has proposed a metal powder for metal laser-sintering comprising an iron-based powder (chromium-molybdenum steel powder, alloy tool steel powder) and at least one kind of nonferrous powder selected from the group consisting of nickel, nickel-based alloy, copper and copper-based alloy, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-152204. The applicant of the present invention also has proposed a powder mixture for metal laser-sintering comprising an iron-based powder (chromium-molybdenum steel), at least one of nickel powder and nickel-based alloy powder, at least one of copper powder and copper-based alloy powder and graphite powder, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2004-277877. The chromium-molybdenum steel and the like are used for the reason of strength and toughness. The copper and copper-based alloy powders are used for the reason of wettability and fluidity. The nickel and nickel-based alloy powders are used for the reason of strength and machinability. The graphite powder is used for the reason of the reduction in the absorbing rate of the light beam and micro cracks.
However, even the metal powders for metal laser-sintering described in Japanese Unexamined Patent Publications (Kokai) Nos. 2001-152204 and 2004-277877 have such problem that the hard iron-based powder adherent to the surface of the shaped object produced by the metal laser-sintering process causes a machinability resistance to be increased upon the finish machining of the surface, which leads to a short lifetime of the machining tool. On the other hand, a slower machining rate is required for extending the lifetime of the tool, resulting in an increase of the necessary time for machining operation.